From BMI to TMI: revisiting adiposity and fitness assessment in young active adults through a historical and contemporary lens

Abstract

Background: Traditional reliance on body mass index (BMI) to assess adiposity may misclassify physically active individuals with high lean mass. The triponderal mass index (TMI) has emerged as a potentially more accurate alternative, but evidence in physically active populations is limited. Purpose: To examine the associations of TMI and BMI with bioimpedance-derived adiposity and selected hematological markers in physically active young adults. Methods: In this cross-sectional study, (n = 59) participants (male = 37, female = 22; age 22.63 ± 2.29 years) underwent anthropometry, whole-body bioimpedance and venous blood sampling. TMI and BMI were calculated, and associations with body fat percentage, hemoglobin, hematocrit, neutrophil-to-lymphocyte ratio (NLR), and platelet-to-lymphocyte ratio (PLR) were examined. Normality assumptions were checked; correlations and multiple linear regressions were computed (α = 0.05). Physical activity status followed WHO/ACSM guidelines (≥150 min·week-1 of moderate-to-vigorous activity). Results: TMI showed a stronger positive correlation with body fat percentage than BMI (r = 0.50, p < 0.001 vs. r = 0.38, p = 0.003). BMI was positively correlated with HGB (r = 0.32, p = 0.013) and HCT (r = 0.26, p = 0.046) and negatively correlated with NLR (ρ = -0.27, p = 0.041) and PLR (r = -0.30, p = 0.022). TMI was negatively correlated with NLR (p = -0.27, p = 0.039). In the multivariable model (predictors: BMI, TMI, HGB, HCT, NLR, PLR), HCT was a significant negative predictor of adiposity (β = -0.34, p = 0.021), whereas TMI showed a positive but marginally non-significant association (β = 0.25, p = 0.073). Assumptions and multicollinearity were acceptable. Conclusion: In physically active young adults, TMI relates more strongly than BMI to bioimpedance-derived adiposity and may aid field-based screening. However, athlete-level decisions should await confirmation in sport-specific, longitudinal studies using criterion methods.